1. Field of the Invention
The invention relates to apparatus and methods for carrying out electrode ionization to purify water.
2. Description of the Related Art
Electrode ionization is a process for removing ions from liquids by sorption of these ions into a solid material capable of exchanging these ions for either hydrogen ions (for cations) or hydroxide ions (for anions) and simultaneous or later removal of the sorbed ions into adjacent compartments by the application of an electric field. (See Glueckauf, E., "Electro-Deionization Through a Packed Bed", Dec. 1959, pp. 646-651, British Chemical Engineering for a background discussion.) The hydrogen and hydroxide ions needed to drive the ion exchange process are created by splitting of water molecules at the interface of anion and cation exchanging solids that contact each other in the orientation that depletes the contact zone of ions, when in the presence of an electric field. This orientation requires that the anion exchanging material face the anode and the cation exchanging material face the cathode. The created hydroxide ions enter the anion exchanging material, and the created hydrogen ions enter the cation exchanging material.
The electrode ionization process is commonly carried out in an apparatus consisting of alternating diluting compartments and concentrating compartments separated by anion permeable and cation permeable membranes. The diluting compartments are filled with porous ion exchanging solid materials through which the water to be deionized flows. The ion exchanging materials are commonly mixtures of cation exchanging resins and anion exchanging resins (e.g., U.S. Pat. No. 4,632,745), but alternating layers of these resins have also been described (e.g., U.S. Pat. No. 5,308,467). Ion exchanging materials consisting of woven and non-woven fibers have also been described. (e.g., U.S. Pat. Nos. 5,308,467 and 5,512,173). The compartments adjoining the diluting compartment into which the ions are moved by the applied electric field, called concentrating compartments, may be filled with ion exchanging materials or with inert liquid permeable materials. An assembly of one or more pairs of diluting and concentrating compartments, referred to as a "cell pair", is bounded on either side by an anode and a cathode which apply an electric field perpendicular to the general direction of liquid flow.
The diluting compartments are each bounded on the anode side by an anion permeable membrane and on the cathode side by a cation permeable membrane. The adjacent concentrating compartments are each correspondingly bounded by a cation permeable membrane on the anode side and an anion permeable membrane on the cathode side. The applied electric field causes anions to move from the diluting compartment across the anion permeable membrane into the concentrating compartment nearer the anode and cations to move from the diluting compartment across the cation permeable membrane into the concentrating compartment nearer the cathode. The anions and cations become trapped in the concentrating compartments because the movement of anions toward the anode is blocked by a cation permeable membrane, and the movement of cations toward the cathode is blocked by an anion permeable membrane. A flow of water is set up to remove the ions from the concentrating compartments. The net result of the process is the removal of ions from the water stream flowing through the diluting compartments and their concentration in the water flowing through the concentrating compartments.
Although at times product water from an electrode ionization (EDI) stack may exceed 18 mega ohms, in general, vendors of EDI equipment will only guarantee a much lower water purity. (US Filter Literature No.: US2006). Application of EDI for the preparation of water for high-pressure boilers and semiconductor manufacture therefore requires that mixed ion exchange polishers be used to treat the EDI product water. It would be desirable to have an EDI system that could reliably produce water equal to that of mixed ion exchange beds. If an EDI system were developed that reliably produced a water with conductivity in the range of 18 mega ohms, it would offer very significant advantages in the design of high purity water systems.
In an EDI stack operated at a sufficiently high current density, the ion exchange resin will be so highly regenerated that it should be capable of producing water equivalent to that produced by mixed ion exchange beds. There are two mechanisms responsible for the degradation of the potential performance of EDI stacks. The one with the largest impact is the diffusion of carbon dioxide from the concentrating compartment back into the diluting compartment. If free carbon dioxide is present in the EDI feed, as it generally is, it will also be present in the concentrate, and since it is not ionic it will diffuse freely through the cation membrane back to the diluting compartment. It cannot diffuse through the anion membrane because most of the anion membrane is alkaline and the carbon dioxide would be converted into bicarbonate in the membrane and forced back into the concentrating compartment by the voltage gradient. Even when no free carbon dioxide is present in the feed water, but bicarbonate ion is present, the problem persists. Bicarbonate or carbonate ions are forced by the voltage gradient within the concentrating compartment toward the cation membrane. The boundary layer next to this membrane is acidic, as is the membrane itself. This converts both bicarbonate and carbonate into carbon dioxide, which can diffuse freely into the diluting compartment through the cation membrane. The involvement of carbon dioxide in degrading the purity of EDI product water is described in the patent literature. (e.g., U.S. Pat. No. 5,868,915). Efforts to remedy this problem have centered on increasing the degree and uniformity of regeneration of the anion exchanging resin in the diluting compartment. These efforts have not been successful because the origin of the problem lies, as pointed out above, in the concentrating compartment.
The second, more fundamental mechanism that limits the ultimate purity of the water that can be produced by EDI is the imperfect perm selectivity of ion exchange membranes; i.e., some cations from the concentrating compartment penetrate the anion membrane and some anions penetrate the cation membrane. In both cases the voltage gradient will then force them into the diluting compartment. The Donnan equation (see "Demineralization by Electrodialysis" by J. R. Wilson Butterworth Scientific Publications, 1960, p. 56) predicts that the penetration of co-ions into the membranes decreases with a decrease in the concentration of these ions in the concentrating compartment. The parasitic processes are illustrated in FIG. 1, which shows a single concentrating channel 10, having cation permeable membrane 12, anion permeable membrane 14 oriented as indicated between anode 16 and cathode 18.
It is possible to reduce the impact of these parasitic transfer processes by, e.g., reducing the concentration of ions in the concentrating stream by reducing the fraction of feed water that is recovered. This approach has a relatively small effect at any reasonable water recovery. Another possibility is to divide the EDI system into two sequential parts. The second, polishing part, would operate with a very low concentration of ions in the permeate and thus produce a higher quality water.